A common symptom in heart failure patients with low left ventricle ejection fraction is the build up of fluids in the lungs. As the heart cannot manage to pump away enough of the blood, fluids build up and cause diminished respiratory capacity and increased strain on the heart.
Commercial systems that monitor fluid build up currently measure impedance at a set frequency. However, to improve the assessment of fluid build up a more complete characterisation of the tissue using several frequencies has been suggested.
It is well known that the Cole-Cole model is a good approximation of human tissue properties in the frequency band between 10-1000 kHz and this model has seen extensive use in body composition estimation algorithms, and some applications in lung tissue characterisation and cardiac assessment (for example as disclosed in US 2006/0247543).
To research the possibilities of including such measurements in a home monitoring solution Philips has developed a wearable bioimpedance device which cycles through a set of frequencies and measures the corresponding impedance.
However, bioimpedance is also affected by various other influences, for example how the tissue is distributed between the measurement points, effects of tissue compression, blood perfusion, air inflow etc. These changes are also picked up during the measurement. In fact, recording impedance to measure some of these signals is well-known in the fields of impedance cardiography and impedance pneumography.
A device that captures impedance values at several frequencies will inevitably capture some of these values at different stages of the respiratory cycle. Current methods solve this problem by averaging over several measurement cycles (also referred to as “sweeps”). However this leads to a long measurement time during which the subject is required to be still. The absolute time depends on the speed of the electronics and accepted error levels of the tissue parameters.